This invention relates to the field of seam construction for joining the ends of a length of papermaking fabric to render that fabric endless. More particularly, it relates to a seaming technique for use with a multilayer layer base fabric. Even more specifically, it relates to a helical coil seam for such a fabric.
Seamed papermaking fabrics, that is, those having seams that may be assembled and disassembled on a papermaking machine without the requirement of stitching or weaving, have been available but have presented problems, primarily in the premature failure of the seamed area. These prior art fabrics can be divided into two basic categories, the first having seams formed outside of the weaving loom and the second having seams formed in the weaving loom.
In the first category of fabrics, those having seams formed outside the weaving loom, the fabrics have generally been flat woven with an independent seam structure attached to the ends of the fabric, such as by sewing a woven tape onto the fabric or piercing it with clipper hooks. These structures have provided poor caliper and density profiles in the seam area. Other structures, such as Gisbourne, U.S. Pat. No. 4,244,084, have formed a gap near the end of the fabric with the fabric end then folded back over a helical coil to lock the seam loops into the fabric. This structure again provides poor caliper and density profiles in the seam area due to the fold back thickness, and the strength and life of the seam is limited to the strength of the stitching holding the folded fabric. These problems have effectively precluded the successful use of any of these types of fabric seams in the wet press section of papermaking machines.
In the second category, in which a pin seam is formed during the weaving process on the loom, the conventional approaches have constructed such a seam by forming loops on two ends around holding cords and then weaving the yarn back into the fabric body. This seaming technique has suffered disadvantages in that the base fabric composition, construction and thickness have been dictated by the requirements of loop formation, as distinguished from papermaking considerations. This has required two layers of machine direction yarn that are capable of being heat set or resin impregnated to be stiff enough to form loops. Such construction has provided an improved seam compared to the first category but has still suffered many problems. These problems include installation difficulties because the seam loops are difficult to mesh together because of inconsistent size, shape and orientation as a result of the weaving method. Also, these seams tend to pull apart, due to machine direction yarn failure. This type of failure has resulted from the requirement of stiff yarns for loop formation, which yarns have intrinsically poor fatigue resistance and low elasticity and resiliency. Another significant problem relates to the requirement that the base fabric thickness be dictated by the method of loop formation and not be designed for optimal water handling and drainage. This frequently results in poor sheet dewatering, reduced paper machine efficiency, reduction in paper quality and a short operational life of the fabric. An additional problem relates to the substantial additional cost in weaving these difficult fabrics, resulting in press felt fabrics so woven being as much as 30% more expensive than comparable, nonseamed press felts.